Apparatus for converting linearly polarized radiation into linearly polarized radiation having a plane of polarization varying linearly with time

ABSTRACT

An improved apparatus for converting linearly polarized radiation having an arbitrary plane of polarization into linearly polarized radiation in which the orientation of the plane of polarization changes linearly as a function of time from the initial arbitrary orientation is discussed, in which apparatus the radiation passes at least thrice through a birefringement element, at least one of the elements traversed being an electrooptical crystal. It is shown that by using a retrodirective element the number of birefringement elements can be reduced and also the voltage to be applied to the electro-optical crystals can be considerably lower.

35 0-385 SR 1 A) OR 3963005 K, m 7' 4 y O V '7 United States n 7 MM [1113,630,595

[72] Inventor Theodom Hendrlkus Peek 3,432,223 3/1969 Uchida 350/150Emmalngel, Elndhoven, Netherlands OTHER REFERENCES [21] P Takasaki,Photometric Measurement of Polarized Light [22] Filed Feb. 2, 1970 byMeans of ADP Polarization Modulatorl .I.O.S.A. Vol. 51, [45] PatentedDec. 28, 1971 731 Assignee U.S.Phillpl Corporation 4 463 Takasaki,Photometric Measurement of Polarized Light New Yorlt,N.Y. r n by Meansof ADP Polarization Modulator ll J.O.S.A. Vol. [32] Priority Feb.3,1969sl N 4(A I96!) 463 331 Netherlands [31] 6901722 Primary Examiner-DavidSchonberg Assistant ExaminerPaul R. Miller Anamey- Frank R. Trifari [54]APPARATUS FOR CONVERTING LINEARLY pounuzan RADIATION INTO LINEARLYPOLARIZED RADIATION HAVING A PLANE OF ABSTRACT: An improved apparatusfor converting linearly POLARIZATION VARYING LINEARLY WITH polarizedradiation having an arbitrary plane of polarization TIME into linearlypolarized radiation in which the orientation of the 3Chlma,2Dr-win Fplane of olarization changes linearly as a function of time I f h p l bd d h h rom t einitia ar itrary orientation is iscusse inw ic ap- [52]U.S. Cl 335513715507, params the radiation passes at least thricethrough a [51] mt Cl G02 1/26 birefringement element, at least one ofthe elements traversed being an electro-optical crystal. [50] Field ofSearch ll5507, It is shown that by using a Ietmdirective element thenumber of birefringement elements can be reduced and also 5 ReferencesCited the voltage to be applied to the electro-optical crystals can beUNITED STATES PATENTS 3,239,671 3/1966 Buhrer 350/150 PATENTED 0EC28 zenENTOR.

INV THEODORUS H. PEEK AGEN APPARATUS FOR CONVERTING LINEARLY POLARIZEDRADIATION INTO LINEARLY POLARIZED RADIATION HAVING A PLANE OFPOLARIZATION VARYING LINEARLY WITH TIME An apparatus for convertinglinearly polarized radiation having an arbitrary plane of polarizationinto linearly polarized radiation the orientation of the plane ofpolarization of which varies linearly with time."

The invention relates to an improved apparatus for converting linearlypolarized radiation having an arbitrary plane of polarization intolinearly polarized radiation the orientation of the plane ofpolarization of which varies linearly with time from the initialarbitrary orientation, in which apparatus the radiation passes at leastthrice through a birefringent element, at least one of the elementstraversed being an electro-optical crystal, while the relativeorientations of the elements and the electric voltages applied to thecrystals have been suitably chosen.

Such an apparatus has been proposed in U.S. Pat. No. 3,558,215, FIGS. Iand 4 of which each show an embodiment. A disadvantage of the apparatusshown in FIG. I of the said application is that it requires threeelectro-optical crystals and comparatively high voltages. To the middlecrystal there is applied a voltage having an amplitude which is twicethat of the voltage applied to each outer crystal. The apparatus shownin FIG. 4 has the disadvantage that the voltage applied to the crystalis comparatively high.

It is an object of the present invention to provide an improvedapparatus of the kind described in the above-mentioned patentapplication. The improvement consists in that a series arrangement ofbirefringent elements is followed by a retrodirective element so thatthe radiation emerging from the series arrangement after being reflectedtraverses the series arrangement in the opposite direction. Thus, theseries arrangement needs only comprise at least two birefringentelements. In addition, the voltage applied to the crystals can be lower.

The invention will be described with reference to the accompanyingdiagrammatic drawing, in which:

FIGS. I and 2 show, by way of example, embodiments of apparatusesaccording to the invention.

In FIG. 1 natural radiation emitted from a source 1 of radiation iscollimated by a lens 2 and converted by a polarizer 3 into linearlyplane polarized radiation. For simplicity only one ray of the beam ofradiation is shown. After passing through a semitransparent mirror 4 theplane polarized radiation passes through the series arrangement of twoelectro-optical crystals 5 and 8 which exhibit the Pockels effect andthe principal directions of which, which are indicated by arrows 6 and9, are at an angle of 45 to one another.

An alternating voltage V,=V sinw! from an alternating voltage source 7is applied to the crystal 5, and an alternating voltage V,=V cosmt froman alternating voltage source 10 is applied to the crystal 8. Thevoltages V and V, are applied so that the electric fields generated bythem in the crystals 5 and 8 respectively are parallel to the directionof propagation of the light in the respective crystal.

The radiation emerging from the crystal 8 is reflected at a plane mirrorII. The radiation then passes through the crystals 8 and 5 in theopposite direction and is reflected by the semitransparent mirror 4 to aphotoelectric detection system I2. In the drawing the beam reflected atthe mirror I] has again been shown by one ray which for clarity had beenshifted relative to the ray incident on the mirror 11.

The amplitude V of the voltage applied to the crystal 5 is such thatlinearly polarized radiation incident on the crystal 5 is converted intocircularly polarized radiation, for the value of V is chosen to be suchthat a phase difference of a quarter wavelength is produced between twooscillations which are created from linearly polarized light when itpasses through the respective crystal.

The amplitude V' of the voltage applied to the crystal 8 is chosen to besuch that a phase difference of a half wavelength is produced betweenthe two oscillations which are created from linearly polarized lightwhen it passes through the crystal 8 at a voltage of2 v.

Since the birefringence of an anisotropic element, in particular anelectro-optical crystal. is linearly added after reflection of theradiation traversing the element, the crystals 5 and 8 together withtheir respective mirror images with respect to the reflector II may eacheffectively be considered as one crystal.

The apparatus shown in FIG. I has the same properties as that shown inFIG. I of the U.S. Pat. No. 3,558,2l5 for the crystal 5 of the newarrangement corresponds to the crystal 4 of the main application, thecrystal 8 together with its mirror image corresponds to the crystal 5,and the mirror image of the crystal 5 corresponds to the crystal 6 ofthe main application.

However, the amplitude of the voltage applied to the crystal 8 is onlyone half of that applied to the crystal 5 of FIG. I of the mainapplication. In addition, the phases of the alternating voltages appliedto the crystal 5 and to its mirror image automatically are identical.Hence, no adjusting difficulties will occur.

In an embodiment in which the crystals 5 and 8 consisted of potassiumdideuterium phosphate (KDDP), V=V'=4 kv. The wavelength A of theradiation was 6,328 AU.

Obviously, in general an apparatus according to the main applicationwhich includes (Zn-l) crystals (where n 2) can be replaced by anapparatus which includes n crystals followed by a retrodirectiveelement.

In the apparatus shown in FIG. 2 radiation which is emitted from asource of light 21 and is converted into a parallel beam by a lens 22falls on a polarizer 23. After passing through a semitransparent mirror24 the linearly polarized beam traverses a series arrangement of a M4plate 25 and a Pocltels crystal 27 the principal directions of which,which are indicated by arrows 26 and 28 respectively, are at an angle of45 to one another. I

The radiation emerging from the crystal 27 is reflected at a planemirror 30. The radiation then traverses the crystal 27 and the M4 plate25 in the opposite direction and is reflected to a photoelectricdetection system 31 by the semitransparent mirror 24.

To the crystal 27 there is applied from a source 29 a sawtooth voltagein which the difference between the maximum 7 and minimum values is suchas to produce a phase difference of one-half wavelength between the twooscillations which are created from linearly polarized light on itspassage through the crystal 27.

The crystal 27 together with its mirror image with respect to thereflector 30 may again be considered as one crystal.

The apparatus shown in FIG. 2 has the same properties as the apparatusshown in FIG. 4 of the main application, for the M4 plate 25correspondsto the M4 plate 25 of FIG. 4, the crystal 27 together withits mirror image corresponds to the crystal 26 of FIG. 4 and the mirrorimage of the M4 plate 25 corresponds to the M4 plate 27 of FIG. 4.

The difference between the maximum and minimum values of the sawtoothvoltage applied to the crystal 27 is only one half of the correspondingdifference of the voltage applied to the crystal 26 of FIG. 4 of themain application.

Apart from a plane mirror a so-called cats eye may be used as theretrodirective element. A cat's eye is composed of a lens and a plane orconcave mirror located in the focal plane of the lens.

What is claimed is:

I. An apparatus for converting radiation having an arbitrary plane ofpolarization into linearly polarized radiation having a plane ofpolarization varying linearly with time with respect to an initialarbitrary orientation, comprising polarizing means in the path of a beamof radiation having an arbitrary plane of polarization for convertingthe plane of polarization of the beam into a linearly polarized beamhaving a stationary plane of polarization, a first birefringent elementin the path of the plane polarized radiation from the polarizer, anelectro-optisource of alternating voltage applied across theelectro-optical crystal.

2. An apparatus as claimed in claim 1 wherein the birefringent elementcomprises a M4 plate.

3. An apparatus as claimed in claim 1 wherein the birefringent elementis an electro-optical crystal.

i i l t t

1. An apparatus for converting radiation having an arbitrary plane ofpolarization into linearly polarized radiation having a plane ofpolarization varying linearly with time with respect to an initialarbitrary orientation, comprising polarizing means in the path of a beamof radiation having an arbitrary plane of polarization for convertingthe plane of polarization of the beam into a linearly polarized beamhaving a stationary plane of polarization, a first birefringent elementin the path of the plane polarized radiation from the polarizer, anelectro-optical crystal in the path of the radiation from the polarizerand having a principal direction displaced 45* with respect to theprincipal direction of the birefringent element, a retrodirectiveelement in the path of the radiation passing through the birefringentelement and the electro-optical crystal and oriented with respect to thebeam of radiation in such a direction as to reflect the radiation backthrough the electro-optical crystal and the birefringent element, and asource of alternating voltage applied across the electro-opticalcrystal.
 2. An apparatus as claimed in claim 1 wherein the birefringentelement comprises a lambda /4 plate.
 3. An apparatus as claimed in claim1 wherein the birefringent element is an electro-optical crystal.